Solid or non-wound golf balls are generally divided into two piece balls and multilayer or three piece balls. Two piece balls are generally more popular with the recreational golfer because they provide a very durable ball while providing maximum distance. Two piece balls are typically made with a single solid core, usually formed of a crosslinked rubber, which is encased by a cover material. The solid core is typically made from polybutadiene, which is chemically crosslinked with zinc diacrylate and/or similar crosslinking agents. The resiliency of cores can be improved by increasing the cross-link density. As the cross-linking increases, however, the compression also increases making the cores harder, which is undesirable. The cover tupically comprises tough, cut-proof blends of one or more ionomers such as SURLYN®, which are resins sold commercially by DuPont or IOTEK®, which is sold commercially by Exxon.
The combination of the above-described core and cover materials provides a “hard” ball that is resistant to cutting and other damages caused by repeatedly striking the ball with a golf club. Further, such a combination imparts a high initial velocity to the ball, which results in increased distance. Due to their hardness, however, these balls have a relatively low spin rate which makes them difficult to control, particularly on shorter approach shots. As such, these types of two piece balls are generally considered to be “distance” balls, but are hard when struck with a club.
Softer cover materials such as balata and softer ionomers in some instances, have been employed in two-piece construction balls in order to provide softer balls and increased back spin rates. With softer cover materials a more skilled golfer can impart back spin on the ball that will make the ball stop once it strikes the landing surface instead of bouncing forward. The ability to impart back spin onto a golf ball is related to the degree that the golf ball cover deforms when it is struck with a golf club. Regardless of the form of the ball, players generally seek a golf ball that delivers maximum distance, which requires a high initial velocity upon impact. Therefore, in an effort to meet the demands of the marketplace, manufacturers strive to produce golf balls having initial velocities as close to, but still within, the maximum velocity allowed by the United States Golf Association (“USGA”) test (250 ft/s±2.5% (5 ft/s)). Furthermore, golf ball manufacturers are also continually searching for new ways in which to provide golf balls that deliver the maximum performance in terms of both distance and spin rate for golfers of all skill levels.
Relatively recently, a number of golf ball manufacturers have introduced multilayer or three piece golf balls, i.e., balls with multiple core layers and/or multiple cover layers, in an effort to overcome some of the undesirable aspects of conventional two piece balls, such as hardness, while maintaining the positive attributes of these golf balls, such as high initial velocity and distance. Examples of such improved multiple layer balls include the Titleist® Pro-V1, among others.
Examples of multiple layer ball patents include U.S. Pat. No. 5,072,944, which discloses a three-piece solid golf ball having a center and outer layer, which are prepared from a rubber composition, preferably having a base polybutadiene rubber. This patent also discloses that it is desirable that the center core be softer than the outer layer, wherein the layers have a hardness of 25-50 Shore C and 70-90 Shore C respectively. U.S. Pat. No. 4,625,964 relates to a solid golf ball having a polybutadiene rubber core of a diameter not more than 32 mm, and a polybutadiene rubber intermediate layer having a specific gravity lower than that of the core material. U.S. Pat. No. 4,848,770 discloses a non-wound three-piece golf ball, which includes a core with a highly filled synthetic rubber or polymeric material, an intermediate layer of an unfilled synthetic rubber and a cover. The core and intermediate layer have a hardness between 50-95. U.S. Pat. No. 5,002,281 is directed towards a three-piece solid golf ball which has an inner core having a hardness of 25-70 Shore C and an outer shell having a hardness of 80-95 Shore C, wherein the specific gravity of the inner core must be greater than 1.0, but less than or equal to that of the outer shell, which must be less than 1.3. U.S. Pat. No. 5,253,871 concerns a golf ball having a three-piece structure comprising an elastomer core, an intermediate layer of a thermoplastic material containing at least 10% of ether block copolymer, preferably blended with an ionomer and a thermoplastic cover.
Several additional patents are more specifically directed to golf balls having multiple cover layers. For example, U.S. Pat. No. 4,431,193 relates to a golf ball having a multilayer cover, wherein the inner layer is a hard, high flexural modulus ionomer resin and the outer layer is a soft, low flexural modulus ionomer resin. Either or both layers may comprise a foamed ionomer resin. U.S. Pat. No. 5,314,187 also relates to golf balls having a cover formed with multiple layers, wherein the outer layer is molded over the inner layer and comprises a blend of balata and an elastomer and the inner layer is an ionomer resin. U.S. Pat. No. 4,919,434 is directed towards a golf ball having a cover, which comprises an inner layer and an outer layer, each of which comprises a thermoplastic resin. Preferably, the layers comprise materials that are capable of fusion bonding with each other. U.K. Patent Application Nos. GB 2,291,817 and 2,291,812 are both directed towards wound golf balls with improved distance comprising a dual cover layer, wherein the inner cover layer has a high hardness as compared to the outer cover layer. These references disclose that the cover layers may be formed from balata or ionomer resins and should have a combined thickness of less than 4 mm. U.K. Patent Application No. GB 2,278,609 discloses a multilayer cover comprising an inner cover layer of a high acid ionomer and an outer cover layer of a soft ionomer or a non-ionomeric thermoplastic elastomer.
Recently, U.S. Pat. No. 5,885,172 (“the '172 patent”) advanced the multiple layer cover design with an inner cover layer made from a hard, high flexural modulus resilient material, e.g., ionomers or ionomer blends and an outer cover layer made from a thin soft thermoset castable reactive liquid material, e.g. thermoset polyurethanes or urethanes, among others. The '172 patent discloses that by creating a golf core and inner cover having a long distance, low spin construction, but adding a thin outer cover layer of a relatively soft thermoset material, the desired back spin for greenside play can be achieved in a distance ball.
There remains a need, however, for an overall soft performance ball that has a soft inner cover, which in combination with a soft, high velocity core produces long distances off of driver clubs, and which in combination with a soft thermoset outer cover also produces high spins off of wedge-type clubs.